The present invention relates to an EL element used for illuminating display units, operation panels or the like in various kinds of electronic apparatus.
EL elements are increasingly used in the sophisticated multi-functional electronic appliances for illuminating the display units and the operation panels. A conventional printing type EL element is described with reference to FIG. 2 and FIG. 3.
FIG. 2 is a cross sectional view of a conventional EL element. The conventional EL element comprises: a transparent insulating film 1 made of polyethylene terephthalate or the like material; a light transmitting electrode layer 2 formed by a sputtering process or an electron beam deposition process covering the whole area of upper surface of the insulating film, or a light transmitting electrode layer 2 formed by printing a transparent synthetic resin containing indium tin oxide or the like material dispersed therein; a light emitting layer 5 formed of a synthetic resin binder 3 containing phosphor 4 of zinc sulfide or the like materials, which emits light, dispersed therein; a dielectric layer 6 of synthetic resin binder containing barium titanate or the like material dispersed therein; a back electrode layer 7 of silver/resin or a carbon/resin composite formed on the dielectric layer 6; and an insulating layer 8 formed of an epoxy resin, polyester resin or the like material. The light emitting layer 5, the dielectric layer 6, the back electrode layer 7 and the insulating layer 8 are overlaid by printing one after the another on the light transmitting electrode layer 2.
An EL element mounted in an electronic appliance is driven by an AC voltage supplied to the light transmitting electrode layer 2 and the back electrode layer 7, the AC voltage is supplied from a circuit of the electronic appliance (not shown). The phosphor 4 contained in the light emitting layer 5 emits light to illuminate display panel, LCD or the like of the appliance from a backside of the display.
When the above-configured EL element emits light in a high humidity environment, a combination of the humidity in the air and the voltage applied sometimes creates a carbonized synthetic resin binder in the synthetic resin binder 3 of light emitting layer 5, which is called a black spot and it impairs the illuminating performance. In order to prevent it, the phosphor 4 of zinc sulfide is generally covered with a moisture barrier layer 4A of metal oxides such as aluminum oxide, titanium oxide, silicon dioxide or the like, and aluminum nitride or the like materials.
In the conventional EL elements, however, if some of phosphors 4 are coagulated with each other when they are treated to be covered with the moisture barrier layer 4A, as shown in FIG. 3(a), the boundary portion 9 between the phosphors 4 may be left uncovered by the moisture barrier layer 4A. Or, when a mixture of the phosphors 4 and the synthetic resin binder 3 dissolved in a solvent are stirred, the moisture barrier layer 4A may get damaged as a result of collision between the phosphors 4, and the phosphor 4 may be exposed as illustrated in FIG. 3(b). Under such circumstance, there is a problem that the metal ion can elude out from the phosphor 4 in the high humidity environment, which leads to a deteriorated electrical insulation with the light emitting layer 5. Thus the black spot phenomenon readily appears.
To address the above-described problem, the inventors of the present application proposed in the Japanese Patent Application No. 2000-196109 to disperse a positive ion exchanger in the light emitting layer 5, so that the ion eluded out of the phosphor in high humidity environment is captured by the positive ion exchanger contained in light emitting layer. In this way, the light emitting layer maintains good insulating property in the high humidity environment even if covering of the phosphor with the moisture barrier layer is incomplete; thus the black spot becomes difficult to appear.
The above described improved EL element works well in so far as it is used in the portable telephone and the like normal electronic apparatus where the voltage applied is within a range of several volts to twenty volts. However, if it is lit at a high brightness for a long time driven by a high voltage e. g. several tens or one hundred volts, the EL element tends to exhibit a problem, or a so-called dark spot. The dark spot is not seen during OFF time, but when the EL element emits light, some area appears darker than the surrounding area. This area is called a dark spot. The dark spot phenomenon is significant among those EL elements in which the light transmitting electrode layer is formed by a sputtering process and formation of the moisture barrier layer of the phosphor is insufficient.
The present invention aims to address the above problem, and provides an EL element of an improved illuminating property where generation of the dark spot is suppressed, besides the suppression of the black spot.
An EL element of the present invention comprises: a light transmitting substrate; a light transmitting electrode layer formed on the substrate; a light emitting layer containing positive ion exchanger; a dielectric layer and a back electrode layer. A dielectric insulation layer is further provided, between the light transmitting electrode layer and the light emitting layer, with a dielectric insulation layer being formed of a synthetic resin that is insoluble with the synthetic resin binder forming the light emitting layer.
The present invention provides an EL element of improved illuminating property, with which the generation of the dark spot is well suppressed, besides the suppression of the black spot.